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Unnaturally long intervals between wildfires and years of drought primed the Sierra Nevada for the explosive conflagration consuming the rugged landscape on the edge of Yosemite National Park, forestry experts say.

The fire had ravaged about 455 square kilometres by Tuesday, the biggest in the Sierra's recorded history and one of the largest on record in California.

Containment increased to 20 per cent but the number of destroyed structures rose to 101 and some 4,500 structures remained threatened. The types of lost buildings were not specified. Firefighters were making stands at Tuolumne City and other mountain communities.

The blaze was just 15 hectares when it was discovered near a road in Stanislaus National Forest on Aug. 17, but firefighters had no chance of stopping it in the early days.

Fueled by thick forest floor vegetation in steep river canyons, it exploded to 4,000 hectares 36 hours later, then to 21,000 hectares and 43,000 hectares within the next two days. On its 11th day it had surpassed 72,000 hectares, becoming the seventh-largest California wildfire in records dating to 1932.

Federal forest ecologists say that historic policies of fire suppression to protect Sierra timber interests left a century's worth of fuel in the fire's path.

"That's called making the woodpile bigger," said Hugh Safford, an ecologist with the U.S. Forest Service in California.

Two years of drought and a constant slow warming across the Sierra Nevada also worked to turn the Rim Fire into an inferno. For years forest ecologists have warned that Western wildfires will only get worse.

The Rim Fire has charred
more than 64,749 hectares (160,000 acres), an area larger than the
city of Chicago. Zoom in for more detail on the fire area as of Aug.
26. (Source: USGS Rocky Mountain Geographic Science Center)

"Every year the summer temperatures are a little warmer, hence the conditions for burning are a little more auspicious," said Safford. "People can deny it all they want but it's happening. Every year the fuels are a little bit drier."

The Rim Fire's exponential growth slowed only after hitting areas that had burned in the past two decades, and Safford says that shows the utility of prescribed and natural burns that clear brush and allow wildfires to move rapidly without killing trees.

"If you look at the Sierra Nevada as a whole, by far the largest portion hasn't seen a fire since the 1910s and 1920s, which is very unnatural," said Safford, who has authored several papers on the increasing wildlife severity across California's mountain ranges. "This one isn't stopping for a while."

Since a 1988 fire affected nearly one third of Yellowstone National Park, forestry officials have begun rethinking suppression policies. Yosemite has adopted an aggressive plan of prescribed burns while allowing backcountry fires caused by lightning strikes to burn unimpeded as long as they don't threaten park facilities.

"Yosemite is one of the biggest experimental landscapes for prescribed fire and it's going to pay off," Safford said. "The Rim Fire is starting to hit all those old fire scars."

The 565-kilometre-long Sierra Nevada is a unique mountain system in the United States. Its Mediterranean climate means four-to-six months of drought every summer. California's mountain flora is designed to burn and even flourish and regenerate healthier after a fast-moving fire.

Instead the Rim Fire is killing everything in its path. The understory ignites trees, and wind is sweeping the fire from treetop-to-treetop in 100-metre walls of flame.

Wildlife worries

Scientists expect the impact on wildlife to be severe. The fire has encompassed nearly the entire migratory range of deer in the region, and the burning treetops likely displaced many of the remaining 300 members of a subset of Great Gray Owl along the Yosemite border, said Daniel Applebee of the California Department of Fish and Wildlife.

"Because their population is so small, any loss is significant," Applebee said.

The fire also cut through habitat of the Pacific fisher, a weasel-like animal that is listed for state and federal protections. The fire has fragmented its range, likely leaving it nowhere to expand, Applebee said.

The Rim Fire is the first of any ecological significance in about a decade in the area stretching from the Sequoia National Forest south of Yosemite to north of Lake Tahoe, said Chad Hanson, a forest ecologist and environmental activist who has published a number of papers on the significance and increasing rarity of post-fire habitat in the Sierra Nevada.

Eventually the forest will come back.

"Because we are in such tremendous deficit of this post-fire habitat type, especially in this area, the Rim Fire is a good thing ecologically," Hanson said. "This is not destruction, this is ecological restoration."

The fire approached the main reservoir serving San Francisco, but fears that the inferno could disrupt water or hydroelectric power to the city diminished. On Tuesday the fire moved into the watershed, which increases the chances of sediment runoff this winter.

Yosemite crews continue to keep water on two groves of giant sequoias less than 16 kilometres from the fire's front lines.